After using conventional pumping techniques very large amounts of oil in a given reservoir remain unrecovered. In an attempt to recover this vast quantity of unpumped petroleum many enhanced oil recovery (EOR) techniques have been developed. The water flooding method is a very common EOR technique that has been in use for some time. Water flooding is a secondary oil recovery technique that is chiefly of importance when the natural production of a well has ceased--that is, when petroleum can no longer be pumped from the well economically using conventional pumping techniques. The term "secondary recovery" as used herein, refers to all petroleum recovery operations used in such areas when spontaneous production of the well can no longer be effected. It includes what is sometimes known in the industry as "tertiary recovery," which is a later stage which begins when the petroleum reservoir is substantially "flooded out" and a large amount of water may be produced before any oil is recovered. Thus, primary recovery is when a well spontaneously flows using conventional pumping techniques and secondary recovery begins when primary recovery is no longer feasible and continues for as long as there is any petroleum in the well which can be economically or feasibly removed.
The water flooding technique comprises injecting water into a petroleum deposit through at least one input well (injection well), thereby causing the petroleum to flow from that area for collection through at least one output well. In the simplest recovery method a number of wells are drilled on the circumference of a circle and a final well is drilled in the center. Water is then pumped into one or more of the wells, typically the ones on the circumference, under high pressure and forced through the petroleum-bearing formations, usually porous rock strata. The petroleum remaining in the strata is forced out by the oncoming water and removed through the output well, usually the one at the center of the circle. More typically an array of injection and production (output) wells are established over an oil field in a manner that will optimize this secondary recovery technique by taking into account the geological aspects of that particular field. Ideally, the water should displace 100 percent of the petroleum in the oil field.
Even though water may pass through a deposit, the inherent incompatibility of oil and water, variation in reservoir rock, including permeability variation, faults and shale barriers may result in some regions of the reservoir rock being by-passed so that large oil bearing areas in the deposit remain untouched. This results in less than 100 percent of the residual oil in the reservoir being recovered. The ability of water, or any other fluid, to displace oil is related to that fluid's mobility ratio. Every fluid has a specific mobility in an oil deposit, which can be defined as the ease with which that fluid flows through a porous medium divided by the viscosity of that fluid. A mobility ratio is the ratio of the mobility of two fluids: for example, oil and water. If a fluid flows much more easily than oil through a reservoir, it will readily bypass oil deposits within the reservoir rather than pushing them toward producing wells. Thus, fluids with low mobility ratios are greatly preferred for enhanced oil recovery applications. Recovery by water flooding techniques is greatly facilitated if the mobility of the petroleum relative to the injection water is at a maximum. This is frequently accomplished by increasing the viscosity of the aqueous medium and decreasing the viscosity of the petroleum, by the addition of suitable chemical agents. Thus, a thickener is ordinarily added to the water while a thinning agent may be charged into the petroleum.
High molecular weight (above about 1,000,000) water soluble polymers are generally added to the injection water used in EOR applications to improve the mobility ratio of the water to the oil. A very large increase in water viscosity can be obtained when certain polymers are added in minor amounts (100 ppm to 1500 ppm). Two general types of polymers are currently being used as injection water viscosifiers., they are polyacrylamides and polysaccharides. In general, partially hydrolyzed and anionic polyacrylamides are used, but cationic polyacrylamides have also been used in a limited number of cases. The mobility ratio improvement obtained using polyacrylamides decreases with water salinity and divalent ion concentration. Therefore, a fresh water source (total dissolved solids less than 10,000 ppm) has traditionally been necessary for the effective use of polyacrylamides in EOR applications as viscosifiers. The environment into which the polyacrylamide solution is injected has traditionally also been required to be substantially free of salts in order to be effective.